FCW L119A2 MWS Conversation Kit 
 
L119A2 CNC Rail + Barrel Nut with Outer Barrel And Unicron 265 Inner Barrel
L119A2 DE GBB Grip with CTR Stock
KAC300 Style Sight Set  
QD Sling Ring, SF Flash Hider, L119A2 Cocking Handle Latch  
You only need one M4 MWS GBBR to transform it into an L119A2-style GBBR